Compositions based on polyvinyl chloride (PVC) and other halogenated polymers, such as chlorinated polyethylene (CPE) and chlorosulfonated polyethylene (CSPE), are commonly used to insulate metal and other electrical conductors. Insulated products include residential and commercial building wire; portable power cord and fixture wires; automotive primary (harness) wire; telephone and other communications wires; instrument and appliance wires; motor lead wires, etc. Compositions based on halogenated polymers typically provide required flame resistance, oil resistance, and resistance to environmental factors such as ozone and ultraviolet light to a greater extent than would similar nonhalogenated polymers (e.g., olefin polymers and copolymers). Therefore, the use of compositions based on halogenated polymers in wire and cable is widespread.
A common feature of all compositions based on halogenated polymers is the presence of metal compounds that serve as heat stabilizers to capture HCl liberated during processing of the composition into its final shape. This processing requires shear at elevated temperatures, often as high as 150.degree.-200.degree. C., that invariably results in loss of HCl. It is necessary to scavenge HCl and to terminate heat and shear-initiated degradative reactions to produce suitable articles of commerce, including extruded wire coverings. The final product of the heat stabilization reaction is a metal chloride.
In addition to providing resistance to mechanical damage and attack by various media, the wire covering must be suitable electrically; that is, must be a good insulator. A typical test of effectiveness as an insulator is to measure volume resistivity of the composition, the unit resistance of the insulation to passage of electric current, per ASTM Standard D257. The volume resistivity of insulating compositions can be reduced by many orders of magnitude by physical influx of small amounts of water. The ingredients used in such compositions are chosen (as far as is practical) to be hydrophobic. The resistance of insulating compositions to loss of volume resistivity through water influx or absorption is commonly investigated by determining this property before and after 24 hours of immersion in water at 70.degree. C. Most polymer compositions that contain standard metal stabilizers result in a severe loss of volume resistivity when tested per ASTM standard D257. For example, when a vinyl halide resin is stabilized with stearate salts of zinc, cadmium, tin, calcium, strontium, barium and antimony, volume resistivity of the polymer after hot water immersion is found to be greatly reduced.
Presently, the wire and cable industry has relied on insulating vinyl halide resins containing heat stabilizers based on lead compounds. Thus, electrically insulating polyvinyl chloride (PVC) compositions usually include dibasic lead phthalate, dibasic lead phosphite, tribasic lead sulfate, or lead stearate, among others, as heat stabilizers. With chlorinated polyethylene or chlorosulfonated polyethylene, litharge and red lead oxide are also used.
It is increasingly perceived that widespread use of lead compounds by the wire and cable industry can pose hazards to workers involved in product fabrication, and to the environment. Thus it is widely viewed as desirable to replace lead-based heat stabilizers in vinyl halide resins with others based on metals having a much lower order of toxicity. Heretofore this has not been possible because of the loss of insulating properties caused by the inclusion of less toxic heat stabilizer compounds of zinc, calcium, barium or tin, for example, in the polymer compositions.